It is desirable to raise cooking chambers to their operating temperature as quickly as possible while suffering minimum heat losses, thereby reducing energy consumption.
With known apparatus, the rise in temperature is relatively long because large quantities of heat are lost by conduction to the frame of the apparatus, and these heat losses result in a temperature increase of the outside walls of the apparatus. Standards are laid down which impose maximum permissible temperatures for the outside walls or cladding of cooking apparatus when in use. For example the temperature may not exceed 100.degree. C. for free-standing apparatus, nor 90.degree. C. for built-in apparatus. It is thus necessary to evacuate the heat lost from the cooking chamber by interposing chimney ducting between the frame and the outer cladding to promote air circulation which may be by natural convection or else forced if a fan is incorporated.
French patent publiction No. 2 193 180 (CEPEM) describes a cooking chamber delimited by a muffle having heat insulation on its outside surface, the muffle is itself located in a frame which also has heat insulation on its outside surface and to which an outer cladding of sheet metal is fixed. This design which uses three layers of sheet metal (the muffle, the frame, and the cladding) nonetheless suffers from some of the thermal drawbacks mentioned above, since the assembly points between the three layers constitute heat bridges and the heat lost by the chamber still needs to be evacuated by chimney ducting located between the frame and the outer cladding. This design also suffers from mechanical problems in that manufacture is relatively complex because of the large amount of sheet metal working and enamelling.
French Pat. No. 946 923 describes an electric oven comprising a muffle covered by a sealed box of fibro-cement which is made independently from the muffle and which is subsequently placed around it. This method of assembly leads to poor insulation and to poor mechanical strength. Further, it does not lend itself to robotization.
Preferred embodiments of the present invention avoid the thermal drawbacks of the prior art by providing a cooking chamber apparatus which does not have heat bridges, thereby limiting heat losses from the chamber by conduction.
Such preferred embodiments also have reduced problems of mechanical strength by eliminating some of the delicate manufacturing operations that have previously been required. This is achieved by assembling the main components of the apparatus (chamber, heat insulation, frame, cladding) as late as possible during manufacture. One result is simplified handling which means that the earlier assembly stages could well be robotized. Fitting other sub-assemblies, eg. doors and hobs, is very easily performed around the frame of preferred embodiments of the present invention.